1. Field of the Invention
The present invention is generally related to processing of metals and alloys, and is more specifically concerned with detection of defects generated near the surface of a workpiece during processing.
2. Description of the Related Art
Advanced metal processing methods are continuously being developed to enable economical manufacture and repair of parts with improved physical properties and often complicated shapes. For example, laser cladding (also called laser powder deposition) is being developed for build-up of stainless steel, titanium and other metals (from metallic powders) to enable near net shape manufacturing and repair of critical parts. Advanced joining methods include laser welding and friction stir welding. The friction stir approach, which involves passing a rotating tool through a solid metallic material, may also be used for friction stir processing (FSP) to locally create a fine-grain microstructure providing improved mechanical properties [F. D. Nicholas, Advanced Materials Processes 6/99, 69 (1999)].
Advanced metal processing typically occurs at high speeds and often involves expensive workpieces and materials so that rapid feedback on the quality of the processed region is critical to controlling scrap rates and costs. Defects that may occur within processed regions of metals include voids, pores, bondlines (incompletely formed bonds), disbonds and cracks. Ideally, such defects would be detected in-line during metal processing so as to minimize scrap and improve product quality via timely corrective action, which could include adjusting processing parameters and/or interrupting the process. Metal processing defects often occur below the surface of the processed region where they cannot be detected by optical, spectroscopic or laser profilometer techniques. Conventional ultrasonic detection methods are sensitive to such subsurface defects but require that the inspected workpiece be in contact with a fluid, which is not practical for in-line use. Inspection methods requiring physical contact between the workpiece and a probe are generally impractical for in-line defect detection. In addition, surface irregularity and roughness typical of processed metal surfaces tend to produce noise signals that interfere with ultrasonic detection based on piezoelectric or EMAT transducers, as well as other conventional methods.
Subsurface defect detection is also a critical requirement for the inspection of cast and forged metals, including ingots and railway rails, for example. Typical defects in ingots and castings include pores and inclusions. Typical defects in railway rails include cracks, which need to be detected in-service.
Laser ultrasonic methods have been developed for non-contact detection of defects in solid parts. Since the “bottom” surface of a part is often inaccessible during machining or processing operations, the most useful laser ultrasonic methods involve both generation and detection on the “top” surface of the part. In this case, a pulsed generation laser beam incident on the part surface at a predetermined generation spot generates ultrasonic waves that propagate within and along the surface of the part. The propagated ultrasonic waves, including those reflected from defects and the bottom surface of the part, are detected via a detection laser beam incident on the part surface at a predetermined detection spot. The propagated ultrasonic waves produce a temporal displacement of the part surface at the detection spot, which is measured via an interferometer that analyzes a portion of the detection laser beam reflected from the part surface. Ultrasonic waves reflected from defects may be distinguished from other reflected ultrasonic waves from the difference in time of arrival of the waves at the detection spot.
Laser ultrasonic methods involving generation and detection on the same surface have been applied to detection of various defects, including voids and cracks, in parts of varied shapes and comprising various materials. These methods have typically involved generation and detection of bulk ultrasonic waves, namely compressional waves, which tend to travel along the surface normal, and shear waves, which tend to travel at angles to the surface normal. Laser ablation produces strong compressional and shear waves, whereas compressional waves produced thermoelastically are relatively weak. Bulk ultrasonic waves are well-suited for detecting defects that are relatively distant from the generation-detection surface. However, bulk ultrasonic waves are not well-suited for detecting near-surface (i.e., subsurface) defects for which the delay time for waves reflected from defects is very short, making ultrasonic measurements difficult.
In addition, application of prior art laser ultrasonic methods has generally been limited to smooth and relatively even surfaces to avoid speckle noise associated with surface roughness and unevenness. In contrast, metallic surfaces processed by laser cladding, friction stirring or other methods tend to be uneven and relatively rough. Consequently, prior art laser ultrasonic inspection methods cannot be directly applied to detection of defects in processed metallic workpieces.
The limitations of prior art laser ultrasonic methods are particularly acute for defect detection during laser cladding. The cladding is typically applied in thin layers and each new layer needs to be inspected for defects before it is buried under subsequently applied layers. This requires detection of subsurface defects that are very near the top surface, which cannot be accomplished using the bulk ultrasonic waves generally employed in the prior art.
The present invention utilizes Rayleigh waves (surface acoustic waves) to detect subsurface defects in processed metallic surfaces. Rayleigh waves have been used in the prior art for characterization of near-surface material properties and for detection of surface-breaking cracks. U.S. Pat. No. 5,894,092 to Lindgren et al. describes use of transducers to generate and detect Rayleigh waves in order to determine near-surface material properties by measuring the Rayleigh wave velocity as a function of frequency. U.S. Pat. No. 4,274,288 to Tittmann et al. describes use of transducers to generate and detect Rayleigh waves in order to determine the depth of a surface-breaking crack through analysis of the ultrasonic frequencies contained in the detected ultrasonic wave. The transducer-based approach described in both of these prior art patents is unsuitable for use on processed metal surfaces, which tend to be relatively rough and uneven, and cannot be used for in-line monitoring during metal processing. In addition, the Rayleigh wave velocity measurements used by Lindgren are relatively insensitive to metal defects, and do not provide the directional information needed for detection of localized defects. Likewise, the ultrasonic frequency analysis used by Tittmann does not provide the directional information needed to locate subsurface defects.
In contrast, the present invention is based on detection and analysis of scattered Rayleigh waves to detect subsurface defects. The prior art provides no suggestion that scattered Rayleigh waves might be useful for defect detection. Another important aspect of the present invention is the use of laser generation and detection of Rayleigh waves so that the invention can be applied to relatively rough and uneven processed metal surfaces, and may be used for in-line monitoring during metal processing.